Inter- and intraspecific transfer of West Nile virus (family Flaviviridae, genus Flavivirus, WNV) occurred infrequently when donor Culex tarsalis Coquillett fed concurrently on house finches with recipient Culex quinquefasciatus Say and Cx. tarsalis. Five of six of these house finches had WNV in blood samples collected by jugular venipuncture 30–45 min postfeeding, with titers ranging from 2.3 to 4.2 log₁₀ plaque-forming units (PFU)/ml. After 2-wk incubation at 26°C, three Cx. quinquefasciatus and one Cx. tarsalis of 230 blood-fed recipients were infected, of which one Cx. quinquefasciatus was capable of transmission. Our data indicated that infectious female mosquitoes feeding on small vertebrates create a nonpropagative viremia capable of infecting concurrently cofeeding females. The proportion of recipients infected is likely related to the amount of virus expectorated by donor females, the blood volume of the vertebrate host, and the susceptibility of the cofeeding mosquitoes.